RHEV 3.3 - VMs get paused due to "no storage space error"

Issue

• Guest is paused with AttributeError: 'Drive' object has no attribute 'format'
• VM has been paused due low storage space
• A guest is paused after migrating from VDSM 3.2.z (4.10.2-30.1 and earlier) to 3.3.0-3 (4.13.2-0.10)
• The guest had previously ran on host with VDSM 3.3.0-1 (4.13.2-0.6) or VDSM 3.3.0-2 (4.13.2-0.9) installed.
• Following error seen repeating in /var/log/vdsm/vdsm.log on host.

Thread-4311::ERROR::2014-02-18 11:17:00,994::sampling::355::vm.Vm::(collect) vmId=`158d248a-ccc2-4776-891a-c60f8396eeea`::Stats function failed: <AdvancedStatsFunction _highWrite at 0x18c2730>
Traceback (most recent call last):
  File "/usr/share/vdsm/sampling.py", line 351, in collect
    statsFunction()
  File "/usr/share/vdsm/sampling.py", line 226, in __call__
    retValue = self._function(*args, **kwargs)
  File "/usr/share/vdsm/vm.py", line 529, in _highWrite
    self._vm.extendDrivesIfNeeded()
  File "/usr/share/vdsm/vm.py", line 2313, in extendDrivesIfNeeded
    if not drive.blockDev or drive.format != 'cow':
AttributeError: 'Drive' object has no attribute 'format'